Aluminum base alloy and piston made therefrom



Patented Apr. 7, 1931 UNITEEB STATES PATENT? OFFICE ROBERT S. ARCHER, OF LAKEWOOD, AND LOUIS W. KEMPF, OF CLEVELAND, OHIO, AS

SIGN ORS TO ALUMINUM COMPANY OF AMERICA, OF PITTSBURGH, PENNSYLVANIA,

l A CORPORATION OF PENNSYLVANIA ALUMINUM. BASE ALLOY AND PISTON MADE THEREFROM N Drawing,

This invention relates to aluminum base alloys of relatively low thermal expansivity and good mechanical and physical properties.

The importance of aluminum alloys having a low thermal expansivity, and also possessing the mechanical and physical properties which are required in industrial applications, has been emphasized by the continued and widely developing use of these alloys for pistons and similar motor parts. Aluminum has a relatively higher thermal expansion than the other metals, with the exception of magnesium, which are used in the construction of motors and similar devices, and, since operation at an elevated temperature causes the parts of the motor to expand, it is essential that this expansion of any one part be, in degree at least, equal to the expansion of other parts. The development of an aluminum base alloy having a satisfactory low thermal expansivity has been retarded by the fact that apparently a low expansivity could only be had at the expense of the physical and mechanical properties which make the aluminum alloys suitable for use as pistons. A piston must, as is well known, possess the physical properties of a fairly high hardness and a reasonable degree of toughness. Furthermore, the alloy of which it is formed should, as a desirable mechanical property, be easily machined, because machining is a necessary operation in the final dimensioning of a cast piston. Heretofore, an aluminum base alloy having a superior combination of the properties of hardness, toughness and machinability, and also possessing a low thermal expansivity, has not been developed.

The objects of this invention are to provide an aluminum. base alloy of relatively low thermal expansion which possesses, in addition to this property, a combination of hardness, strength and machinability favorably comparing with the properties heretofore found only in such alloys of relatively higher thermal expansivity, and to provide a piston made of such alloy.

Aluminum base alloys in whlch SlllCOIl is the predominating alloying constituent, be-

Serial No. 328 ,046.

are ordinarily considered to be the only alloys in which a combination of low thermal 'expansivity and good mechanical properties can be found. However, other alloys, regardless of their objectionable relatively high thermal eXpansivity are used more extensivehardness commonly required in aluminum base alloy pistons.

\Ve have discovered an aluminum base aling present in from about 5 to 15 per cent,

loy in which silicon is the predominating alloying constituent, and which has a low thermal expansivity together with mechanical properties and machining characteristics comparable to other such alloys having relatively high thermal expansivity. According to our invention, this singular combination of properties is produced by the addition of suitable amounts of the alloying constituents nickel, magnesium and copper to the aluminum-silicon alloys. This alumi num base alloy retains the low expansivit characteristic of the binary aluminum-silicon alloys, but combines with'this property a high hardness and good machinability. Although the alloying constituents present in the alloy preferably vary within relatively narrow limits, to meet various conditions under which aluminum base alloys of low thermal expansivity may be used the alloying constituents may be varied within relatively wide limits without impairing the low thermal expansivity or losing the essential hardness of the alloy. By the use of well known heat treatment methods, the hardness of the alloy may be varied within a fairly wide range without materially altering its coefiicient of thermal expansion.

While the alloys which we have discovered find, from their nature and constitution, their largest application in pistons, cylinder heads and similar cast articles, there are alloys, within the limits of composition described hereinafter which are capable of be such as connecting rods. In this and similar applications the alloys, because of their novel properties, are particularly useful.

The limits within which tliese added constituents may vary depends to a large extent upon the desired combination of properties. \Ve have determined that from about 7 to 15% of silicon, 0.2 to 3% of magnesium, 0.5

to 7% of nickel, and 0.3 to 7% 'of copper are the limits within which alloys of practical importance may be formed, the aluminum content usually being not less than about 80%. The silicon content generally lies between about 10 to when the alloy has low thermal expansivity. Less than 10% of silicon may be added under some conditions, but at an increase of thermal expansivity. Similarly, a high content of silicon, even as much as may be present if machinability is not a prime consideration. lVe have found that amounts of silicon between about 10 and 15% usually produce an alloy best suited to commercial practice, but where it is possible to disregard poor machinability, or to use special tools enabling the handling of alloys not commercially machinable with ordinary steel tools, alloys containing as much as about 25% of silicon may be used.

The amount of magnesium added to the alloy also depends: upon the properties desired. The presence of magnesium increases the hardness obtainable by heat treatment, and it has been found that the machinability of the alloy is also improved by magnesium. However, larger amounts than 1 to 1.5% of magnesium do not generally increase these properties over those obtained with smaller amounts, while the brittleness of the alloy is increased by higher amounts of magnesium. Preferably 0.2 to 1% of magnesium is added, and this amount usually produces the best results.

The amount of nickel which is advantageously added to the alloy may vary within a comparatively wide range. lVe have found that nickel improves the machinability of the alloy, and also lowers the thermal expansivity and increases the initial hardness. For some purposes, nicked may be advantageously added in amounts as high as 7% and as low as 0.5%, but we have found that its presence in amounts from 2 to 5% gives the most satisfactory results. Higher contents of nickel, while improving the machinability and further decreasing the thermal expansivity, also tend to embrittle the alloy, and, where strength or toughness is of importance, 9. content higher than 4 to 5% is not desirable. On the other hand, with lower amounts of nicked the advantageous effect of its presence is somewhat impaired.

While aluminum base alloys containing silicon, magnesium and nickel as the added alloying constituents possess properties much superior to the known alloys of low thermal expansivity, it has been found that the prop erties of these alloys may be considerabl improved by the addition of copper. arge amounts of copper materially increase the difiiculties of machining, such for example as the wear on tools, and are for'this reason generally undesirable. However, the, initial hardness, and the hardness of the heat treated alloy, is materially increased to an unexpected degree by the addition of a small amount of copper. For instance, under equal heat treatments an alloy containing 1at% of silicon, about 2% of nickel and 1% of mag nesium may have a Brinell hardness as low as 85, while an alloy containing equal amounts of silicon, nickel and magnesium, together with a small amount of copper, has, under similar conditions, a Brinell hardness 15 to 25 points higher than that mentioned. Such increased hardness is very desirable, because it is obtainable without a decided increase in brittleness and machining difficulties, and without an increase in thermal expansivity. According to this invention, the copper is preferably added in small amounts usually lying between 0.5 to 2%, but Where machining is not the deciding factor, or where special machining tools may be employed. the copper may be added in amounts as high as about 7%.

An aluminum base alloy which we have found to be especially adaptable to commercial needs for pistons contains about 14% of silicon, 2% of nickel, 1% of magnesium and 0.75% of copper. Other alloying constituents such as iron, which is usually found as an impurity in high silicon alloys, may be present in the amounts usually found in high grade commercial ingots wthout appreciably atl'ecting the essential properties of the alloy. In general, it is desirable to use a good grade of commerical aluminum in making the alloy. However, grades of lower purity may be used, as may aluminum of extremely high purity. The absence or presence of the impurities commonly found in high grade commercial aluminum apparently does not cause a substantial change in the essential properties.

The coeflicient of thermal expansion of the preferred alloy is about 19 x 10' per degree centigrade for the temperature interval between 20 and 100 C. In comparison, the

coefiicient of expansion of an aluminum base alloy containing about 10% of copper is 22 x 10 or approximately 15% higher than that of the alloy here provided] In fact our improved alloy has a thermal expansivity considerably lower than any of the well known aluminum base alloys of similar mechanical and machining properties.

An aluminum base alloy containing 14% of silicon, 2% of nickel, 1% of magnesium and 0.7 5% of copper has a Brinell hardness of about 85 in its cast condition. This hardness may be increased by suitable heat treatment to about 125 to 130, a hardness substantially equal to or exceeding that of the aluminum base alloys of high thermal expansivity. For instance, a sample of the last-mentioned alloy was subjected to a solution treatment for 2 hours at 530 (1., thereafter rapidly cooled to room temperature, and subsequently aged by reheating to between 125 and 150 C. for a period of about 25 hours, after which the alloy had a Brinell hardness of 129.

The aluminum base alloy containing silicon, nickel, magnesium and copper provided according to this invention has many distinct advantages over the aluminum base alloys heretofore used in the manufacture of pistons and similar parts. It possesses a low thermal expansivity combined with a hardness such as is required for pistons and similar parts. Furthermore, it may be commercially machined, thus eliminating one of the principal disadvantages of the aluminumsilicon and aluminum-copper-silicon alloys which have heretofore been used where low thermal expansion and high hardness were desired.- An added advantage of the alloy here provided is that it is substantially (about 10% for the preferred composition) lighter about 1% of magnesium,'a.nd about 0.75% of copper.

3. A piston formed from an aluminum base alloy contaihing from about 7 to 15% of 51110011, from about 0.2 to 3% of magneslum, from about 0.5 to 7% of nickel and from about 0.3 to 7% of copper.

4;. A piston formed from an aluminum'base alloy containing about 14% of silicon, about our names.

ROBERT S. ARCHER. LOUIS W. KEMPF.

than the 10% copperalloy most widely used for pistons.

In the foregoing, the addition of alloying elements other than silicon, nickel, magnesium and copper has been mentioned. It is sometimes necessary to add certain amounts of other alloying constituents to modify the properties of the alloy to makeit particularly suited for articles other than pistons and similar parts. For instance, manganese, chromium, titanium and similar alloying elements may be added to produce certain special pro erties which may be desired in addition to those possessed by the alloy here provided. Similarly, other alloying constituents may be added to adapt the alloy to a special purpose without destroying the essential properties of low thermal expansivity and high hardness.

According to the provisions of the patent statutes, wehave described the principle of ourinvention, and have given specific examples of it. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced by the provision of alloys other than those specifically described.

We claim as our invention 1. An aluminum base alloy containing from about 7 to 15% of silicon, from about 0.2 to 3% of magnesium, from about 0.5 to 7% of nickel and from about 0.3 to 4% of copper.

2. An aluminum base alloy containing about 14% of silicon, about 2% of nickel,

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